The perception of color by human vision involves the impact of light of different wavelengths in the visible spectrum (400 nm-780 nm) on the human eye, and the processing of the resultant signals by the human brain. For example, in order for a typical individual to perceive an object as “red”, light in the range of wavelengths of about 580-780 nm must be reflected from the object onto the retina of the eye of the individual. Depending upon the spectral distribution of the light and assuming normal color vision, the individual perceives different colors from a wide range of such colors.
In addition, the individual perceives various characteristics of the color. The color itself is also termed the “hue”. In addition, saturation determines the purity of the color, such that a color which is saturated is perceived as highly vivid, while a pastel version of the same color is less saturated. The combination of hue and saturation forms the chrominance (chromaticity) of the color. As perceived by the individual, color also has brightness, which is the apparent or perceived energy of the color, such that the color “black” is actually the absence of brightness for any color.
Although color is a complex combination of physical and physiological phenomena, it has been found that colors can be approximately matched by combinations of only three colors, usually red, green and blue.
The following discussion considers the reproduction of color through various types of media, in particular with regard to a comparison of color reproduction through electronic display devices to color reproduction through physical printed material, such as colored inks printed onto paper, for example. The first section of the “Background” discusses color reproduction on physical printed material. The next section of the “Background” discusses such color reproduction on electronic display devices. The final section of the “Background” discusses the effect of color reproduction on electronic display devices on the process of printing colored inks onto physical printed material.
Section 1: Printed Material
Color images can be presented on substrates such as slides, films, and paper, and also on electronic displays. Color reproduction on paper involves subtractive color mixing. The term “subtractive” refers to the creation of color by removing a portion of the spectrum of light transmitted to the eye.
The nature of the color system for printed material is predicated upon the optical properties of the materials, particularly of the inks, although the paper or other material onto which the ink is placed also has an effect with regard to a color reflection characteristic, such that the material reflects light differently according to its reflection spectrum. Inks or dyes applied in printing behave as filters that pass only part of the white light spectrum. The light incident on the paper is spectrally filtered by the ink layer and reflected back towards the observer. Four types of inks are typically used, although of course other types of ink systems can also be used: Cyan (C), Magenta (M), Yellow (Y) and Black (K). The transmission spectra of “ideal” CMY ink filters are shown in FIG. 1D. Each of the primary inks blocks its complementary color, such that C passes green and blue and blocks red, M passes red and blue and blocks green and Y passes red and green and blocks blue. the black ink blocks the whole spectral range. The spectra of the complementary “ideal” RGB are shown in FIG. 1B. Thus, upon reflection from the paper surface only part of the spectrum arrives to the eye of the viewer creating the sensation of a unique color.
The intensity of the light reflected from the ink layer is measured through a filter of the complementary color. Assuming a perfect complementary filter, namely one with a 100% transmission in the relevant range and zero elsewhere, the ink layer density D, which measures its spectral blocking properties is given by:
                              D          C                =                ⁢                  -                                    log              10                        ⁡                          [                                                                    ∫                    Red                                    ⁢                                                            I                      ⁡                                              (                        λ                        )                                                              ⁢                                                                                  ⁢                                          ⅆ                      λ                                                                                                            I                    o                                    ⁢                                                            ∫                      Red                                        ⁢                                                                                  ⁢                                          ⅆ                      λ                                                                                  ]                                                                        D          M                =                ⁢                  -                                    log              10                        ⁡                          [                                                                    ∫                    Green                                    ⁢                                                            I                      ⁡                                              (                        λ                        )                                                              ⁢                                                                                  ⁢                                          ⅆ                      λ                                                                                                            I                    o                                    ⁢                                                            ∫                      Green                                        ⁢                                                                                  ⁢                                          ⅆ                      λ                                                                                  ]                                                                        D          Y                =                ⁢                  -                                    log              10                        ⁡                          [                                                                    ∫                    Blue                                    ⁢                                                            I                      ⁡                                              (                        λ                        )                                                              ⁢                                                                                  ⁢                                          ⅆ                      λ                                                                                                            I                    o                                    ⁢                                                            ∫                      Blue                                        ⁢                                                                                  ⁢                                          ⅆ                      λ                                                                                  ]                                          
Note that by the use of the complementary filter, the measured intensity I(λ) gives the amount of light passing through the blocking region of the spectrum of the ink, and thus is a measure of its blocking properties. The higher the density, the more saturated is the color. For low density, the amount of light in the “blocked” region of the spectrum is high and comparable to that in the transparent area, and thus a non-saturated color is obtained.
Most printing methods are binary in nature, namely an ink layer of a certain thickness is either present or absent on the paper surface. To obtain “gray levels” for each of the inks, halftone printing is applied. The paper is divided by a virtual grid to printing dots. The area of each printing dot is partially covered with ink. The relative area covered by ink in the printing dot is known as the dot area or dot percentage (dot %). If the paper is only partially covered with ink, the apparent density is lower than the density of a solid ink layer.
For example, consider a cyan ink, which passes Blue (B) and Green (G) and blocks Red (R). The cyan solid ink density is the amount of R passing through a full coverage cyan layer. If there are small dots of cyan on paper, which are so small so that they are below the eye resolution, the paper has a pale cyan tint (a “gray level” or gradation of cyan). The apparent density of this tint is lower than that of a solid cyan layer because more red color is received by the eye, since a large amount of the red component of the light is received from the uncovered areas of the white paper. If the density of the tint area is defined in a similar manner as the way that the density of the inks are defined, there is a relationship between the tint density, the solid ink density, and the relative area of inked paper to the relative area of non-inked paper. This relation is called the Murray-Davis relation:
      dot    ⁢                  ⁢    %    =            1      -              10                  -                      D            t                                      1      -              10                  -                      D            S                              
Here Dt and Ds are the tint and the solid ink layer densities respectively. The Murray-Davis relation has a physical basis, but in many cases it gives inaccurate results. Yule and Nielsen suggested a similar relation to obtain the dot % from the apparent density and the solid ink density, which fits better the experimental results, but does not have a direct physical explanation. The Yule-Nielsen formula reads as:
      dot    ⁢                  ⁢    %    =            1      -              10                              -                          D              t                                /          n                            1      -              10                              -                          D              S                                /          n                    where n is an empirical value usually equal to 1.5.
The dot % is determined according to the value of the relevant pixel in the image file. Files in CMYK format, designated for printing, specify the amount of each of the primary color in 8-bit format, corresponding to 256 gray levels. A value of zero corresponds to zero coverage of the relevant ink, while a value of 255 corresponds to a full coverage of relevant ink in the printing dot.
Each of the inks is layered according to its virtual grid. When examining the printed paper at the usual viewing distance, the impression of color is achieved. However, looking at the printed paper through a magnifying glass resolves a delicate arrangement of dots in the original primary colors, and overlap regions of colors. The elementary colors, seen through the magnifying glass, include the four primaries CMYK, the three overlaps between two primaries giving Red (overlap of M and Y), Green (overlap of C and Y) and Blue (overlap of C and M), and the white color of the paper (see FIG. 1C). Overlap of CMY gives a black color, and any overlap of C, M or Y with black gives also black. Thus, the total number of elementary colors is seven, CMY RGB and white/black (white/black may be considered the same color at different brightness levels). Since the CMY RGB and white/black dots may not be not discernible to an unaided eye, the eye integrates (additively) the light reflected back from them, creating the sensation of color.
Section 2: Electronic Devices
Color is also presented by electronic meals, for example by display devices such as computer monitors, televisions, computational presentation devices, electronic outdoor color displays and other such devices. These systems involve additive color mixing of three primaries: red, green and blue. The mechanism for color display may use various devices, such as Cathode Ray Tubes (CRT), Liquid Crystal Displays (LCD), plasma display devices, Light Emitting Diodes (LED) and three-color projection devices for presentations and display of video data on a large screen, for example. The term “additive” refers to the creation of color by combining light of at least two spectra before transmission to the eye. The spectra of “ideal” RGB primaries are shown in FIG. 1B, and the construction of other colors by additive mixing is shown in FIG. 1A. In practice, however, ideal primaries do not exist.
As an example of the operation of such a device, CRT displays contain pixels with three different phosphors, emitting red, green and blue light upon excitation. In currently available displays, the video signal sent to the display typically specifies the three RGB color coordinates (or some functions of these coordinates) for each of the pixels. Each coordinate represents the strength of excitation of the relevant phosphor. An individual viewing the display integrates the light coming from neighboring colored pixels to get a sensation of the required color. The process of integration is performed by a combination of the physiological activity of the eye itself and of processing of signals from the eye by the brain, without individual awareness of the process.
Although color is a complex combination of physical and physiological phenomena, it has been found that colors can be approximately matched by combinations of only three colors, usually red, green and blue, a finding which has been exploited by various types of electronic display devices. These three colors are the additive primaries. The match is perceptual, and depends on the processing of the spectrum of light arriving to the eye, by the human vision system and the brain. By combining different amounts of each color, a wide spectrum of colors can be produced. Nevertheless, not all colors can be produced by electronic display devices, since some combinations require negative values of one or more of the primaries. Although these negative values are allowed mathematically, they cannot be realized.
Therefore, these systems cannot display the full range of colors which are available to the human eye, because some colors are presented by negative values of one or more of the primaries, which cannot be realized by a physical light source. Certain background art electronic devices and systems use a fourth “color”, which is actually light passed through a neutral filter, or “white light”, and which is used for controlling brightness of the displayed color, as described for example with regard to U.S. Pat. No. 5,233,385. However, the use of the neutral filter does not affect the ultimate spectrum of colors which can be displayed.
Section 3: Proofing of Printed Material on Electronic Devices
Reproduction of color involves the creation of an accurate apparent color match between original and reproduction. Color originals may be, for example, pictorial slides, which are analog in nature. They have a very large gamut, larger than typical reproduction means, such as offset print. In the age of digital information most of the reproduction process is done digitally. The original slide is scanned to obtain a file containing the color data in terms of RGB values. The file is converted to CMYK separations, and then plates are created, which are installed on a press for print. To obtain color consistency, proofs are performed and examined in various stages of the process, to assure that each step is color consistent with its previous step.
While in one embodiment of the system and method of the present invention, CMYK data is converted, in other embodiments other input data may be converted, having other forms or formats. Furthermore, embodiments of the system and method of the present invention may be used to proof various ink systems, such as ink systems not based on CMYK inks. For example, certain ink systems include CMYK ink plus additional numbers of inks, and other ink systems do not use the CMYK inks.
Accurate presentation of color is very important for printed matter. In order to achieve good color match, the image is currently proofed by printing a “hard proof” on paper, and sending this paper “hard proof” to the customer and/or designer for approval. Upon approval, the proof is delivered to the printing shop, where the printer working on the press machine must then adjust the press machine until the printed sheets match the hard proof.
This manual procedure limits the advantages of digital workflow. The need for an accurate digital “soft proof” on an electronic display is clear.
Currently available “soft proofing” devices enable designers and pre-press personnel to view the works on a computational device such as a personal computer or workstation displays (usually based on Cathode Ray Tubes, or CRT), while the final product is a printed image on paper. However, these background art devices do not overcome inherent deficiencies for digital print proofing, and in particular do not provide good color match, in the sense that they cannot accurately replicate the colors electronically as they would appear on the printed material. This is a serious drawback, as many printed works are now transferred digitally from design to printed material over a network, and any procedure which must be performed through printing onto physical material, before the final printing step, significantly reduces the efficiency of the printing process.
It is important to understand that RGB color displays generally cannot provide good color match to printed image for various reasons. The transition from the additive RGB color space of an electronic display to the subtractive CMYK color space of printing is rather problematic. In principle each of the subtractive primaries (except for black) should transfer two of the additive primaries and block the third one (see FIG. 1). However, in practice, this may not be the case for several reasons. First, and most important, the spectra of the RGB phosphors used in displays, and the inks used in printing are far from that of ideal primaries. Second, in a CRT, there is no overlap between the primary RGB colors, while in printed material there are regions of overlap between inks, increasing the number of elementary colors that the eye integrates.
Third, the spectra of the light passing through the CMYK inks depends on the lightning conditions, namely the spectrum of the white light which illuminates the paper of the printed material. Furthermore, even two printing machines may generally provide different printing results (in terms of color), since various properties of the ink and the paper play major roles in the resulting color sensation.
It is quite clear that the effects discussed above cause the color gamut of a printing press to be different from that of a display, and there are certain regions in each of the color gamuts which are not represented in the other gamut.
Even if the gamut of the print is embedded in that of the display, it is still required to provide transformation from CMYK value to RGB value in such a way that an apparent match is achieved. This is the basis of existing methods of color matching in general, and “soft proofing” on displays in particular. They are based on mapping the color space of the output device (printing press, display) into device-independent color space known as L*a*b*, defined by CIE. Using this mapping, a multi-dimensional transformation from the RGB space of the display into the L*a*b* space can be performed. Then, another transformation from the L*a*b* space into the CMYK space of the printing press is performed. These transformations, known as profiles, are performed by a color management system on the data file containing the work, before printing. The International Color Consortium (ICC) standardized this method for color matching.
It is important to note that the above method is based on an approximate apparent match, which therefore depends on various factors including illumination conditions, and not on a full spectral match, which is always valid. The profile creation process involves many mathematical calculations and data processing, as well as good mapping of the relevant color spaces. In particular, the transformation from the three-dimensional L*a*b* color space to the four-dimensional CMYK color space is not unique, such that extra parameters are required for its determination. The profile creation process is thus quite cumbersome, and fails to give good results in many cases. Furthermore, it requires a creation of profiles for each type of paper/ink/machine/illumination condition combination. This limits the use of the ICC workflow in the industry.
Thus, existing “soft proofers” based on software implemented mathematical transformation are only approximate, and cannot produce an accurate spectral match. In many cases, even an apparent color match is difficult to achieve. This behavior is of course unacceptable to customer, resulting in the procedure of color proofing with a “hard” proof, which is better able to simulate the subtractive and spectral nature of printed material.
A more useful solution would enable a direct spectral mapping to be performed between the color spectrum of the printed inks as they appear on the printed material and the colors of the electronic displayed image, such that these colors would be spectrally matched. Such a solution would enable the viewer to accurately determine the appearance of the image as printed on the material, such as paper, through the electronic display, such as a display device for a computer, for example. Unfortunately, such a solution is not currently available.
Therefore, there is an unmet need for, and it would be highly useful to have, a device, system and a method for accurate electronic display of an image to be printed with inks on printed material such as paper, such that the colors of the printed material have an accurate spectral match to the displayed colors on the electronic device, such as a computer monitor for example, in order to provide “soft” proofing of an image before being printed.